DESCRIPTION This project further characterizes a very interesting regulating ensemble in E. coli discovered in the principal investigator's laboratory. This ensemble is a redox-sensitive SoxRS system which consists of a redox-sensor SoxR protein and an inducible proximal activator, SoxS. Upon oxidation by superoxide or nitric oxide, SoxR is activated and, in turn, it activates SoxS. SoxS induces transcription of a large number of genes involved in antioxidant defenses as well as antibiotic resistance. In the current proposal the investigators want to establish whether another environmentally important oxidant, ozone, triggers the SoxRS system. Preliminary experiments performed in the foreign investigator's laboratory showed that SoxR is, indeed, activated by the exposure of the cells to ozone. Further experiments are planned in order to characterize the time- and dose- dependence of this activation. Another set of preliminary data suggests that vitamins C and E have an inhibitory effect on the SoxRS system. This phenomenon will be investigated further with respect to dose-dependence. One of the most interesting observation made about the SoxRS system is that its activation leads to antibiotic resistance of E.coli. This aspect of the system will be investigated in ozone-activated airborne bacteria in Mexico City.